1. Field of the Invention
This invention relates generally to implantable devices, such as cardiac stimulators, designed to be situated within a living body and to exchange information with devices located outside the body. More particularly, the invention relates to a novel technique for processing and exchanging data between an implantable device and a remote device which makes efficient use of power and signal processing capabilities within the implantable device.
2. Description of the Related Art
In recent years increasingly sophisticated systems have been developed for monitoring and controlling certain physiological processes via implanted devices. Such devices are typically placed within a patient""s body and remain resident within the patient""s body over extended periods of time. One such device, commonly referred to as a cardiac stimulator, is commonly implanted in a patient""s chest region and includes circuitry both for monitoring the functioning of the patient""s heart as well as for providing stimulus for the heart when needed.
Conventional implantable cardiac stimulators include one or more electrical leads which extend between electronic circuitry provided within the device housing and portions of the patient""s heart. For example, leads extending from the stimulator may be terminated in the right atrium and right ventricle of the patient""s heart to provide both sensing and stimulation capabilities. The circuitry is programmed to execute desired functions, such as monitoring, stimulating, and storing of diagnostic or other data. A power supply is implanted with the device to furnish the electrical energy needed for its operation.
Through their relatively short history, cardiac stimulators and other implantable devices have experienced very considerable evolution. For example, early cardiac stimulators provided fixed rate stimulating pulses designed to regulate the patient""s heart beat only. Later designs, sometimes referred to as xe2x80x9cdemandxe2x80x9d pacemakers, also offered heart monitoring capabilities, providing stimulating pulses only as needed based upon the monitored functioning of the heart. Further improvements in cardiac stimulators included programmable rate pacemakers, dual chamber pacemakers, and xe2x80x9crate-responsivexe2x80x9d pacemakers, each providing increased flexibility and adaptability of the monitoring and stimulation functions to more closely conform to the needs and physiological parameters of the patient, such as the patient""s level of physical activity.
Throughout the evolution of cardiac stimulators and other implantable devices, a persistent problem has been the efficient provision and use of electrical energy. In general, the power source, typically including a specially designed electric battery, is implanted with the electronic circuitry to provide all power necessary for the monitoring, stimulation, programming and other functions of the implantable device over extended periods of time, often measured in years or decades. To provide the longest possible life to the implanted power source, therefore, it is generally a goal in the design of such devices to reduce the power needed for all aspects of their function. For example, the replacement of early fixed rate pacemakers with demand pacemakers significantly reduced the energy continuously dispensed by the device by generating stimulating signals only as needed, thereby prolonging the effective life of the power source. Other developments have also extended the useful life of such power sources, although further improvements are still needed.
A particularly useful function of implantable devices involves the ability to transmit and to receive information between the implantable device and an outside programming or monitoring unit. Data exchange between the implantable device and the external unit permits parameters, such as physiological data, operational data, diagnostic data, and so forth, to be transmitted from the implantable device to a receiver from which the data can be accessed and further processed for use by an attending physician. The data is particularly useful for gaining insight into the operation of the implantable device as well as the state of the patient""s organs and tissues. The ability to exchange data in this manner also permits the physician to reprogram or reconfigure the implantable device as may be required from time to time due to evolution of the patient""s condition.
Data exchange between an implantable device and a remote, outside device is often accomplished by xe2x80x9cwaveform telemetryxe2x80x9d in which the data is conveyed through the patient""s tissue and skin. Early waveform telemetry systems employed in implantable cardiac stimulators transmitted signals through analog encoding. For example, in one known type of pacemaker, analog samples representing operational or physiological parameters are transmitted as the pulse position of a radio-frequency pulse train. The pulse train is output by either the implantable device or the outside device, and is interpreted or decoded upon receipt by the other device. While such techniques are extremely useful for gaining access to information relating the performance of the patient""s organs and of the implantable device, analog telemetry circuits typically yield low resolution and often AC-coupled and uncalibrated signals, effectively limiting their utility and reliability.
To address the shortcomings of analog telemetry systems, digital telemetry schemes have been developed. For example, certain digital telemetry systems are presently in use wherein a radio-frequency carrier or radio-frequency pulse train is modulated by digital information corresponding to samples of the analog signals to be telemetered. Such digital data communication methods make use of an analog-to-digital (A/D) converter for transforming samples of analog signals into digital format for transmission. If multiple analog signals are to be transmitted, an analog signal multiplexer is employed to select one signal at a time to feed to the A/D converter. A programmer or a telemetry system controller selects the channel from which the next sample is to be converted prior to transmission. However, such processing reduces the sampling rate per signal due to the relatively large portion of time and telemetry channel bandwidth which must be used for communicating the channel information. Moreover, a relatively fast A/D converter is required because the telemetry system must wait for the conversion to be completed before being able to transmit the data. The use of a fast A/D converter results in considerable energy usage, reducing the effective life of the implantable power source.
Alternatively, a predetermined data acquisition sequence may be established to eliminate the need for continuously communicating the channel to be converted. This alternative, however, limits the flexibility of the system as the number and identity of channels to be transmitted generally cannot be changed without first reconfiguring the sequencer. Moreover, this technique requires the sampling process to be synchronized with read operations executed by the telemetry circuit, as asynchronous operation may yield transmission or reception of invalid or misinterpreted data.
There is a need, therefore, for an improved technique for exchanging data between an implantable device and a device external to a patient""s body. There is a particular need for a telemetry technique which is capable of transmitting digitized data to and from an implantable device, but which avoids certain of the drawbacks of existing systems as summarized above.
The present invention provides a novel technique designed to respond to these needs. The technique permits the exchange of information between an implantable device and an external device, and the conversion of analog information to digital information according to and at rates adapted to conform to the needs and desires of a user of the external device, typically an attending physician. The telemetry technique enables the effective transmission of analog signals, such as intracardiac electrograms, intracardiac and spacial impedance signals from the implantable device to an external device via high speed digital telemetry. In an advantageous configuration, the technique employs dedicated registers in the implantable device for storing data corresponding to digitized values of analog signals associated with the registers. The contents of the registers may be telemetered to the external device upon demand. In a preferred arrangement, the contents of the registers are updated automatically each time the register is read, refreshing the stored data contained in the register as a function of the read requests received from the external device. The A/D conversion process, its sequence and its speed are advantageously determined by the requests of the external device in real time, providing enhanced flexibility and reduced energy consumption, while offering the attending physician the most up-to-date information on the specific information desired to be accessed.
Thus, in accordance with a first aspect of the invention, a data telemetry system is provided for transmitting signals from an implantable device to a remote external device. The implantable device is configured to collect data representative of at least first and second operational parameters of the implantable device or a biological system in which the implantable device is disposed. The telemetry system includes first and second memory circuits, a telemetry circuit, and a control circuit. The memory circuits allow for storage of values representative of the first and second parameters, respectively. The telemetry circuit is coupled to the first and second memory circuits, and is configured to transmit first and second signals representative of the first and second values. The signals transmitted by the telemetry circuit are in response to transmission request signals from the remote device. The control circuit is coupled to the first and second memory circuits and is configured to control replacement of the first and second values in the first and second memory circuits in response to transmission of the respective first and second signals. An analog-to-digital conversion circuit is advantageously coupled to the first and second memory circuits and converts analog signals to the first and second values in response to transmission of the corresponding value via the telemetry circuit. A switching circuit may be provided for applying analog signals to the conversion circuit as the first and second values are telemetered.
In accordance with another aspect of the invention, an implantable device is provided which is configured to be disposed in a living body. The device includes a signal processing circuit, a signal conversion circuit, memory circuits, a telemetry circuit, and a control circuit. The signal processing circuit detects at least two operational parameters of the device or the body, and generates analog parameter signals representative thereof. The signal conversion circuit is coupled to the signal processing circuit for converting the analog parameter signals to digitized parameter values. The memory circuits store the digitized parameter values produced by the conversion circuit. The telemetry circuit transmits signals representative of the digitized parameter values in response to request signals received from an external unit. The control circuit is coupled to the signal processing circuit and is configured to apply analog parameter signals to the conversion circuit in response to transmission of the digitized values. The control circuit may advantageously control the conductive state of switches in a switching circuit for selectively applying the analog parameter signals to the conversion circuit in coordination with the telemetry of the digitized values.
In accordance with still another aspect of the invention, a system is provided for telemetering digital data from an implantable medical device to an external device. The system includes a data acquisition circuit, an analog-to-digital converter, a telemetry circuit, and a control circuit. The data acquisition circuit is configured to generate analog parameter signals representative of operational parameters of the implantable device or a body in which the implantable device is disposed. The analog-to-digital converter is coupled to the data acquisition circuit for converting the analog signals to digital values. The telemetry circuit transmits digital values produced by the converter to the external device in response to request signals from the external device. The control circuit selectively applies the analog signals to the converter. The digital values are thus telemetered to the external device in a sequence and at a rate defined by the request signals in real time.
The invention also provides a method for transmitting data between an implantable device configured to be disposed in a living body and an external device disposed outside the body. In accordance with the method, first and second analog parameter signals are generated which are representative of operational parameters of the body or of the implantable device. The analog parameter signals are converted to digital values, and the digital values are stored in a memory circuit. One of the digital parameter values is telemetered to the external device in response to a request signal from the external device. The analog parameter signal corresponding to the telemetered parameter value is then converted to an updated digital value. The telemetered parameter value is then replaced in the memory circuit with the updated digital value.
In accordance with a further aspect of the invention, a method is provided for acquiring data representative of cardiac function. The method includes the steps of monitoring a plurality of parameters representative of cardiac function in an implantable device, and generating analog parameter signals representative thereof. The analog parameter signals are converted to respective digital parameter values. The digital parameter values are stored in a memory circuit. A desired digital parameter value is telemetered to an external device in response to a request signal from the external device. The analog parameter value corresponding to the desired digital parameter value is then converted to an updated digital value, and the desired digital parameter value is replaced in the memory circuit with the updated digital value. The analog parameter signals may be derived from sensed signals, such as in a dedicated signal processing circuit. The method may be repeated to obtain effective sampling rates for the parameters as defined by the request signals from the external device. Sampling rates may be different for different parameters depending upon the particular parameter of interest and the rate of sampling required for obtaining meaningful information on the parameter.
In accordance with still another aspect of the invention, a method is provided for telemetering digital data from an implantable medical device to an external device. According to the method, analog signals are generated which are representative of operational parameters of the implantable device or a body in which the implantable device is dispose. A series of data request signals are transmitted from an external device to the implantable device. The data request signals define a sequence of desired samples of the operational parameters. The analog signals are processed in the implantable device to convert analog signals corresponding to the desired samples to digital values and to telemeter the digital values to the external device in response to the data request signals. The data request signals may advantageously establish effective sampling rates for specific parameters of interest, depending upon the nature of the parameter, and the sampling rate required to obtain meaningful information on them.